Method for fabricating light-emitting devices with vertical light-extraction mechanism

ABSTRACT

A light-emitting device comprises a lattice structure to minimize the horizontal waveguide effect by reducing light traveling distance in the light-absorption medium of the light-emitting devices, and to enhance light extraction from the light-emitting layer. The lattice structure includes sidewalls and/or rods embedded in the light-absorption medium and dividing the light-absorption medium into a plurality of area units. The area units are completely isolated or partially separated from each other by the sidewalls. Also provided is a method of fabricating a light-emitting device that comprises a lattice structure, which lattice structure includes sidewalls and/or rods embedded in the light-absorption medium and dividing the light-absorption medium into a plurality of area units.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/761,708filed on Apr. 16, 2010, the content of which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates in general to light-emitting devices, andmore particularly to light-emitting devices with reduced self-absorptionand enhanced light extraction, and to a method for fabricating the same.

DESCRIPTION OF THE RELATED ART

From light-extraction point of view, light traveling in media constantlyundergoes loss due to the following mechanisms:

-   -   1) Interface loss because of refractive index mismatch and        existence of evanescent wave along the interface.    -   2) Total internal reflection. When a ray of light is incident        from an optically denser medium to a less dense medium, a total        internal reflection takes place at the interface if the angle of        incidence is over a critical angle. Suppose the optically denser        medium has a refractive index n₂, and the less dense one has a        refractive index n₁, the critical angle, θ_(c), is define by the        relationship: sin θ_(c)=n₁/n₂. Because of the total internal        reflection, light can only escape from the optically denser        medium to the less dense medium within a light escape cone (a        cone with a solid angle of 2π(1−cos θ_(c))). The light escape        percentage equals to (1−cos θ_(c))/2, or approximately to θ_(c)        ²/4, or more conveniently approximately equal to (n₁/n₂)²/4.        Here approximation is made when θ_(c) is small.    -   3) Absorption of medium. If medium possesses nonvanishing        absorption coefficient, α, light intensity decays exponentially        with travelling distance t within the medium, exp(−αt).

In recent years, solid-state light sources, such as light-emittingdiodes (LEDs), are increasingly challenging traditional light sourcessuch as incandescent lamps or fluorescent lamps, due to theirtechnological and economical advantages. Currently, high efficient LEDwhite light lamps with efficacy over 100 lm/watt are commerciallyavailable from several independent vendors. Unlike traditional lightsources, solid-state light sources generate light in a solid-statematerial, which usually possesses a refractive index, n, above 2, muchlarger than that of air or free space (n equal to 1). For example, forGaN-based visible LEDs, the light-emitting medium, InGaN, has arefractive index above 2.46 depending on the indium composition. Thismeans that only a small portion of light generated within InGaN canescape from the optically denser medium. From the abovementioned lossmechanism 2), for a thin-film GaN-based visible LED, light has twoescape cones. This translates into a total escaped light portion beingonly about 8% of the total generated light within the InGaN medium. Inview of this poor light extraction efficiency limited by total internalreflection, methods like surface roughening (e.g. U.S. Pat. Nos.7,422,962, 7,355,210, aiming at reducing total internal reflection), LEDchip side-wall shaping (e.g. U.S. Pat. No. 7,652,299, aiming at increaselight escape cones), and photonic crystal incorporation (e.g. U.S. Pat.Nos. 5,955,749, 7,166,870, 7,615,398, 7,173,289, 7,642,108, 7,652,295,7,250,635, aiming at enhancing spontaneous light generation rate andlight extraction for specific wavelengths) were introduced in the priorart.

So far, light loss due to total internal reflection in LEDs has beenpartly taken care of in the prior art. However, light loss due toself-absorption has hardly been addressed. For nitride based LEDs or anyother LEDs heteroeptaxially deposited on foreign substrates, themismatch of refractive index between the substrate and the epilayersintroduces a horizontal waveguide mechanism. This confines part (visiblelight) or most (deep UV light) of the generated light travelinghorizontally in the epilayers. Since the epilayers forming the LEDstructure posses finite or even very strong absorption coefficient, thishorizontally waveguide effect essentially results in large light loss.In the literature, GaN and InGaN were reportedly to have an absorptioncoefficient of 300 cm⁻¹ and 10⁵ cm⁻¹ for visible light, respectively.These numbers mean that for blue-green visible light traveling in GaN,the light loss due to GaN absorption reaches 14% per 5 μm distance. Forblue-green visible light traveling in the active-region InGaN quantumwells, the light loss due to InGaN self-absorption reaches 10% per 10 nmdistance. This kind of huge loss due to self-absorption means that thereis a high need to get out of the light from the light-emitting medium assoon as possible.

SUMMARY OF THE INVENTION

Accordingly, the present invention proposes a solution to solve theabovementioned self-absorption within solid-state light-emittingdevices. A mechanism is introduced to interrupt, or block the horizontalwaveguide effect by reducing light traveling distance in absorptionmedia of the light-emitting devices, and facilitate or enhance lightextraction through the shortest path, which is most likely perpendicularto the light emitting layer, or in the vertical direction. Thisinvention involves introducing vertical light-guiding grid and/or rods,forming a lattice structure vertically embedded in light-generationmedium. The lattice structure, in the form of grid or rod lattice,extracts light and guide the extracted light, through the shortest path(substantially perpendicular to the light-emitting layer), out of thelight-emitting medium to free space for different applications. As thelattice structure significantly reduces the distance traveled by lightin the light-generation medium, the present invention greatly reduceslight self-absorption by the light-generation medium, and in themeantime, also effectively reduces light loss due to total internalreflection.

One aspect of the present invention provides a light-emitting device,which comprises: a substrate with a lattice structure integral with thesubstrate and extending upward from an upper surface of the substrate; an-type layer formed over the substrate; a light-emitting layer formedover the n-type layer; and a p-type layer formed over the light-emittinglayer; wherein the lattice structure penetrates the n-type layer and thelight-emitting layer.

Preferably, the lattice structure comprises a plurality of rods whichpenetrate the n-type layer and the light-emitting layer, and extend upto or into the p-type layer.

Preferably, the lattice structure comprises sheet-shaped sidewalls whichpenetrate the n-type layer and the light-emitting layer, and extend upto or into the p-type layer, and divide the n-type layer and thelight-emitting layer into a plurality of area units.

The lattice structure may have a grading refractive index whichdecreases in the direction from the light-emitting layer to the p-typelayer.

A coating can be formed on the lattice structure, separating the latticestructure from the n-type layer and the light-emitting layer.

There can be a gap formed between the lattice structure and the n-typelayer and the light-emitting layer.

Another aspect of the present invention provides a light-emittingdevice, which comprises: a n-type layer; a p-type layer; anlight-emitting layer sandwiched between the n-type layer and the p-typelayer; and a lattice structure comprising sheet-shaped sidewalls whichpenetrate the light-emitting layer and divide the light-emitting layerinto a plurality of area units.

The area units can be completely isolated from each other by thesheet-shaped sidewalls of the lattice structure. Preferably, the latticestructure also penetrates the n-type layer and divides the n-type layerinto a plurality of area units. A conductive layer can be depositedadjacent to the n-type layer and being in electrical connection witheach of the area units of the n-type layer.

The area units can be partially separated from to each other, and anenclosure degree of the area units can be equal to or higher than 20%.

The light-emitting device can further comprise another n-type layerformed adjacent to one side of the n-type layer opposite to thelight-emitting layer, and another lattice structure can be embedded inthe another n-type layer, so that the two lattice structures arepositioned in different layers. The another lattice structure maycomprise sheet-shaped sidewalls which are vertically embedded in theanother n-type layer. The another lattice structure may comprise aplurality of rods which are vertically embedded in the another n-typelayer.

Another aspect of the present invention provides a light-emittingdevice, which comprises: a substrate with a first lattice structureintegral with the substrate and extending upward from an upper surfaceof the substrate; a first n-type layer formed over the substrate; asecond n-type layer formed over the substrate; a light-emitting layerformed over the n-type layer; a p-type layer formed over thelight-emitting layer; and a second lattice structure vertically spacedfrom the first lattice structure; wherein the first lattice structure isembedded in the first n-type layer and the second lattice structurepenetrates the second n-type layer and the light-emitting layer.

Preferably, the first lattice structure comprises sheet-shaped sidewallsor a plurality of rods which are vertically embedded in the first n-typelayer.

Preferably, the second lattice structure comprises a plurality of rodswhich penetrate the second n-type layer and the light-emitting layer,and extend up to or into the p-type layer, Preferably, the secondlattice structure comprises sheet-shaped sidewalls which penetrate thesecond n-type layer and the light-emitting layer, and extend up to orinto the p-type layer, and divide the second n-type layer and thelight-emitting layer into a plurality of area units.

Another aspect of the present invention provides a method forfabricating a light-emitting device. The method comprises: patterning asubstrate to form a lattice structure extending upward from thesubstrate; forming a n-type layer over the substrate so that the latticestructure penetrates the n-type layer; forming a light-emitting layerover the n-type layer so that the lattice structure penetrates thelight-emitting layer; and forming a p-type layer over the light-emittinglayer so that the lattice structure extends up to the p-type layer orinto the p-type layer.

The method may further comprise forming a coating on the latticestructure before forming the n-type layer, the light-emitting layer, andthe p-type layer. The coating can be formed via epitaxial growth andetching back, or via sputtering, or via oxidation. The method mayfurther comprise removing the coating after forming the n-type layer andthe light-emitting layer, so that a gap is formed between the latticestructure and the n-type layer and the light-emitting layer.

Another aspect of the present invention provides a method forfabricating a light-emitting device. The method comprises: forming amaterial layer over a substrate; patterning and etching the materiallayer to form a lattice structure from the material layer, wherein thelattice structure extends upward from the substrate; forming a n-typelayer over the substrate so that the lattice structure penetrates then-type layer; forming an light-emitting layer over the n-type layer sothat the lattice structure penetrates the light-emitting layer; andforming a p-type layer over the light-emitting layer so that the latticestructure extends up to or into the p-type layer.

The material layer can be single layer or comprise multiple layers ofdifferent refractive index. The material layer may have a varyingrefractive index in a direction perpendicular to the substrate.

The method may further comprise forming a conductive layer between thematerial layer and the substrate before patterning and etching thematerial layer, wherein the step of patterning and etching the materiallayer also etches into a top portion of the conductive layer.

Another aspect of the present invention provides a method forfabricating a light-emitting device. The method comprises: patterning asubstrate to form a first lattice structure extending upward from thesubstrate; forming a first n-type layer over the substrate so that thelattice structure is embedded in the first n-type layer; forming amaterial layer over the first n-type layer; patterning and etching thematerial layer to form a second lattice structure from the materiallayer; forming a second n-type layer over the first n-type layer so thatthe second lattice structure penetrates the second n-type layer; forminga light-emitting layer over the second n-type layer so that the secondlattice structure penetrates the light-emitting layer; and forming ap-type layer over the light-emitting layer so that the second latticestructure extends up to the p-type layer or into the p-type layer.

Another aspect of the present invention provides a method forfabricating a light-emitting device. The method comprises: forming afirst material layer over a substrate; patterning and etching thematerial layer to form a first lattice structure from the materiallayer, wherein the lattice structure extends upward from the substrate;forming a first n-type layer over the substrate so that the firstlattice structure is embedded in the first n-type layer; forming asecond material layer over the first n-type layer; patterning andetching the material layer to form a second lattice structure from thematerial layer; forming a second n-type layer over the first n-typelayer so that the second lattice structure penetrates the second n-typelayer; forming a light-emitting layer over the second n-type layer sothat the second lattice structure penetrates the light-emitting layer;and forming a p-type layer over the light-emitting layer so that thesecond lattice structure extends up to the p-type layer or into thep-type layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention. Like numbers in the figures refer to like elementsthroughout, and a layer can refer to a group of layers associated withthe same function.

FIGS. 1A-1E illustrate steps of a method to fabricate vertical waveguideof a light-emitting device deposited on a substrate by patterning thesubstrate to form a lattice structure.

FIGS. 2A-2C illustrate steps of a method to fabricate vertical waveguideof a light-emitting device by patterning a material layer deposited on asubstrate to form a lattice structure.

FIGS. 3A-3F illustrate steps of another method to fabricate verticalwaveguide of a light-emitting device deposited on a substrate bypatterning the substrate and coating the patterned substrate to form alattice structure.

FIGS. 4A-4B illustrate steps of another method to fabricate verticalwaveguide of a light-emitting device by patterning a material layerdeposited on an electrically conductive layer which in turn is depositedon a substrate.

FIG. 5 is a top view of a two-dimensional lattice structure with closedarea units of square-shape serving as vertical waveguide.

FIG. 6 is a top view of a two-dimensional lattice structure withsemi-closed area units of square-shape serving as vertical waveguide.

FIG. 7 is a top view of a one-dimensional lattice structure withsemi-closed area units serving as vertical waveguide.

FIG. 8 is a top view of another two-dimensional lattice structure withsemi-closed area units serving as vertical waveguide.

FIG. 9 is a top view of a one-dimensional lattice structure withsemi-closed area units having repeating zigzag sidewalls combined withstraight sidewalls: “Y” sidewalls.

FIG. 10 is a top view of a one-dimensional lattice structure withrepeating zigzag sidewalls.

FIG. 11 is a top view of a one-dimensional lattice structure withmodified repeating zigzag sidewalls.

FIG. 12 is a top view of a two-dimensional square lattice structure madeof vertical rods.

FIG. 13 is a top view of a two-dimensional rhombus/hexagonal latticemade of vertical rods.

FIG. 14 illustrates the cross-sectional structure of a light emittingdevice with a lattice structure grown on one side of a substrate andanother lattice structure is formed in the substrate from another sidethereof.

FIG. 15 illustrates the cross-sectional structure of a light emittingdevice with double vertical waveguide formed by two vertically arrangedlattice structures.

FIG. 16 illustrates the cross-sectional view of the light emittingdevice as shown in FIG. 15, but with a lattice structure that hasinclined sidewalls or rods.

FIG. 17 illustrates the cross-sectional view of the light emittingdevices shown in FIG. 15, but with a lattice structure that has inclinedsidewalls or rods.

FIG. 18 illustrates the cross-sectional view of the light emittingdevices shown in FIG. 15, but with a lattice structure that has inclinedsidewalls or rods.

DETAILED DESCRIPTION OF EMBODIMENTS

As mentioned above, the conventional light-emitting devices fabricatedon foreign substrate suffer from horizontal waveguide effect because ofthe refractive index mismatch between the device material and thesubstrate material. The horizontal waveguide effect reduces lightextraction efficiency from two aspects. Firstly, for most of thelight-emitting devices, except for some edge emitting devices, lightemits from top (epilayer) and/or bottom (substrate) side. The horizontalwaveguide effect leads to very large angle of incidence of light, wheretotal internal reflection takes place and reduces light extractionefficiency. Secondly, horizontal waveguide effect eventually enlargeslight traveling distance within the epilayers, and results in largelight loss due to epilayers' self-absorption of the generated light.

Accordingly, the present invention proposes a method to reduce thehorizontal waveguide effect and improve light extraction efficiency.Horizontal waveguide effect happens because of the refractive indexmismatch of materials in the vertical direction (i.e., layer depositiondirection). One aspect of the present invention introduces refractiveindex mismatch in the horizontal direction (i.e., perpendicular to thelayer deposition direction). This mismatch in the horizontal directionresults in vertical waveguide effect, hence greatly reduces the lighttraveling distance and total internal reflection, leading to enhancedlight extraction efficiency.

In some embodiments, such as GaN-based LEDs epitaxially deposited on anoptically less dense substrate, such as sapphire, quartz, spinel, andthe like, the substrate is etched through pre-formed masks, to apredetermined depth, to form a lattice of rods or sheet-shaped sidewallsmade of the same substrate. LED growth is then performed on the etchedsubstrate. The remaining sheet-shaped sidewalls or rods are made withsmall-enough cross-section, that growth of layers on the substrateactually only happens in the etched area, not on top of the rods or thesheet-shaped sidewalls, due to sufficient adatom surface diffusionlength under epitaxy temperatures. The etch depth, or the height of therods/sidewalls is selected to make the rods/sidewalls in the vicinity ofthe LED active-region (light-emitting medium). In some embodiments, therods/sidewalls barely punch through the light-emitting layer. Thesequential p-type layers' growth planarizes the whole structure. In someembodiments, wherein the substrate has a lower refractive index (say,sapphire, n=1.76) compared to that of the light-emitting structure (say,GaN, n=2.46), the difference in the refractive indexes between thelight-emitting structure and the rods/sidewalls implements a verticalwaveguide effect, confining light traveling vertically within thestructure to the surfaces. These vertical light-guiding rods/sidewallsblock the horizontal waveguide effect arising from the refractive indexmismatch between the epilayers and the substrate. In conventional LEDstructures, horizontal dimension is much larger than vertical dimension.For example, the vertical thickness of an LED structure is only a fewmicrons, where the horizontal chip size can be up to millimeters or evenlarger. The transformation of horizontal waveguide effect into verticalwaveguide effect greatly reduces self-absorption loss. At the meantime,the vertical waveguide effect enabling light to strike at the interfacebetween different layers or the medium/air interface with a small angleof incidence, eliminates total internal reflection, and results in anenhanced light extraction efficiency.

In other embodiments, such as GaN-based LEDs epitaxially deposited onsapphire substrate, the substrate is etched through pre-formed masks, toa predetermined depth, to form a lattice of sheet-shaped sidewalls orrods made of the same substrate. The etched substrate is then planarizedby performing insulating nitride (GaN, AlGaN, or InGaN) growth, such asepitaxial growth. The planarized substrate is etched for a second timethrough a second pre-formed masks, leaving the previously formedrods/sidewalls coated with a coating of the insulating nitride layer,with a thickness comparable to the emitted light wavelength in themedium. In this way, a lattice of composite rods or sheet-shapedsidewalls is achieved. LED growth is then performed on the etchedsubstrate with the composite rods or sheet-shaped sidewalls. Thecomposite rods and sidewalls have small enough cross-section in the topview direction, ensuring that growth of layers on the substrate onlyhappens in the etched area, not on top of the sidewalls or the rods, dueto sufficient adatom surface diffusion length under epitaxytemperatures. The etch depth, or the height of the lattice structure isselected to make the rods or the sidewalls in the vicinity of the LEDactive-region, light-emitting medium. In some embodiments, the rods orthe sidewalls barely punch through the light-emitting layer. Thesequential p-type layers' growth planarizes the whole structure. Inthese embodiments, where bigger refractive index contrast exists betweenthe rods/sidewalls and the coating thereon, and smaller refractive indexcontrast exists between the light-emitting layer and the coating, lightis confined in the coating and vertically guided to the emittingsurfaces. Hence, light is extracted immediately after generation awayfrom the light-generation medium, avoiding the strong self-absorption ofthe light-generation layer and horizontal waveguide effect.

In still other embodiments, such as GaN-based LEDs epitaxially depositedon sapphire substrate, the substrate is etched through pre-formed masks,to a predetermined depth, to form a lattice of rods or sheet-shapedsidewalls made of the same substrate. These rods/sidewalls are thencoated with insulating, transparent materials possessing similar or evenlarger refractive index than that of GaN, by means of, say, sputtering.The coating materials can be insulating group-III nitride (GaN, InN,AlN, or any ternaries or quaternaries of them), silicon nitride (n=2.1to 2.3), TiO₂ (n=2.6), et al, with a thickness comparable to the emittedlight wavelength in the medium. In this way, a lattice of compositerods/sidewalls is achieved. LED growth is then performed on the etchedsubstrate with the composite rods. The composite rods/sidewalls havesmall enough cross-section in the top view direction, ensuring thatgrowth of layers on the substrate only happens in the etched area, noton top of the rods or the sidewalls, due to sufficient adatom surfacediffusion length under epitaxial temperatures.

In still other embodiments, wherein light-emitting device structure isdeposited on substrate having similar or larger refractive indexcompared to that of the light-emitting structure, such as GaN-based LEDsepitaxially deposited on nitride (e.g. GaN) substrate, siliconsubstrate, gallium arsenide substrate, silicon carbide substrate, andthe like, the substrate is etched through pre-formed masks, to apredetermined depth, to form a lattice of rods or sheet-shaped sidewallsmade of the substrate. These rods or sidewalls are then coated withinsulating, transparent materials possessing smaller refractive indexthan that of light-emitting structure, by means of, say, sputtering oroxidation. The coated material can be air (n=1), which means there is asubstantial gap between the light-emitting structure and thesheet-shaped sidewalls or rods. The coated materials can also be epoxyresin (n=1.4), or SiO₂ (n=1.46), with a thickness substantially largerthan the emitted light wavelength in the medium. In this way, a latticeof composite sheet-shaped sidewalls/rods is achieved. LED growth is thenperformed on the etched substrate with the composite sheet-shapedsidewalls/rods. The composite sheet-shaped sidewalls or rods have smallenough cross-section, ensuring that growth only happens in the etchedarea not on top of the sheet-shaped sidewalls or rods, due to sufficientadatom surface diffusion length under epitaxy temperatures. In theseembodiments, wherein big refractive index contrast exists between thelight-emitting medium and the coating insulating layer of the compositesheet-shaped sidewalls/rods, light is vertically confined in thelight-emitting structure and is vertically guided to the emittingsurfaces, avoiding the strong self-absorption due to horizontalwaveguide effect. These embodiments can result in enhanced lightextraction efficiency.

The lattice structure of the vertical waveguide sheet-shapedsidewalls/rods can be a square, rhombus, or any other suitable one ortwo-dimensional lattice. The cross section of these rods can be circle,rectangle, triangle, hexagonal, or other suitable shape. The sidesurfaces of the sheet-shaped sidewalls/rods can be vertical, orinclined. The side surfaces can be smooth or rough. The period of thelattice can be adjusted to an optimal number, by compromising thefabrication complexity and the light-extraction effectiveness. And thesheet-shaped sidewalls/rods filling factor, which is defined as theratio between the total volume of the rods or the sheet-shaped sidewallsof the lattice structure in the light-emitting layer or thelight-emitting layer and the volume of the light-emitting layer or thelight-emitting layer, can be optimized, by compromising thelight-extraction effectiveness and the sacrifice of light-emittingmedium. The filling factor can be in the range from 1%-30%, for exampleless than 10%. But the filling factor is not limited to any particularrange as long as the total extracted light with the lattice structure islarger than that without the lattice structure.

This invention because of its uniqueness, allows for the lattice ofsheet-shaped sidewalls/rods to be fabricated in prior to the LEDepitaxial deposition, eliminating drawbacks associated with other lightextraction approaches. For example, in the prior art, photonic crystalapproach is in general thought to be effective to enhance lightextraction. The photonic crystal approach usually etches out holes tomodulate refractive index of the LED, expecting to have enhanced lightextraction efficiency in accordance with the physics of photoniccrystal. However, the photonic crystal fabrication process leaves largedamaged surface areas within the LED. Surface, especially damagedsurface, acts as non-radiative recombination path. When presented in thevicinity of the MQW, these surfaces will reduce the MQW internal quantumefficiency dramatically.

FIGS. 1A-1E illustrate a method of introducing vertical waveguide into alight-emitting device grown on foreign substrate, for example,nitride-based light-emitting diodes (LED) grown on a sapphire substrate.The substrate may have a refractive index smaller than that of theepi-layers in the light emitting device. The method starts with supplyof a substrate 10, forming an etch-protection mask 15 over substrate 10,and etching substrate 10 in the presence of mask 15, via ion-coupledplasma reactive ion etching (ICP-RIE) or any other suitable knownetching methods, to fabricate a lattice structure including plurality ofrods 12 formed from substrate material 10. Rods 12 extend substantiallyin the vertical direction (i.e., layer deposition direction). Thelattice structure also can be made of a plurality of sheet-shapedsidewalls 12′ extending substantially in the vertical direction as shownin FIGS. 5-11. Then the so-prepared substrate 10 is loaded into a growthreactor, such as reactors of metalorganic chemical vapor deposition(MOCVD), molecular beam epitaxy (MBE), hydride vapor phase epitaxy(HVPE), or the like reactor to carry out the device epitaxial growth.The device structure formed by epitaxial growth includes an insulatingbuffer layer 21, an n-type electron supplier layer 22, an light-emittinglayer 23, and a p-type hole supplier layer 24. To function as anelectrical device, finally there are an n-electrode 31, and ap-electrode 32. The p-electrode 32 is preferably to be transparent.

Typically, for nitride-based visible LEDs, the substrate 10 can be atransparent sapphire wafer. The insulating buffer layer 21 can be madeof undoped GaN or AlGaN, with a thickness ranging from 0.1 to 1 μm. Theresistivity of layer 21 is preferably above 100 Ω/cm. N-type electronsupplier layer 22 can be a silicon-doped GaN layer, with a thicknessfrom 1 to 5 μm, and a sheet resistance preferably below 30 Ω·cm. N-typeelectron supplier layer 22 can be a single layer or contain severaln-type layers made of AlInGaN with different bandgap offsets (hencedifferent composition of Al, In, Ga), in order to obtain high electroninjection efficiency into the light-emitting layer 23. Thelight-emitting layer 23 is often made of material(s) of lower bandgapthan those of the n-type electron supplier layer 22 and p-type holesupplier layer 24. Light-emitting layer 23 can be a single layer ofInGaN, or other suitable material conventionally used as light-emittingmedium, or a single or multiple quantum well including well(s) andbarrier(s). P-type hole supplier layer 24 is typically magnesium dopedGaN, with a thickness from 50 to 500 nanometers, and resistivity 0.2 to2 Ω·cm. P-type hole supplier layer 24 can be a single layer or containseveral other p-type layers made of AlInGaN with different bandgapoffsets (hence different composition of Al, In, Ga), in order to obtainhigh hole injection efficiency into the light-emitting layer 23.N-electrode 31 can be made of thin Ti/Al alloys and a thick metal pad,such as Au. P-electrode 32 can be made of thin Ni/Au alloys and a thickmetal pad, such as Au, or a transparent conducting oxide (TCO) layer,such as indium tin oxide (ITO).

Generally speaking, the n-type layer, the light-emitting layer, and thep-type layer in the present invention can be any conventional n-typelayer, p-type layer, and quantum well layer as used in light-emittingdevices or light-emitting diodes.

One aspect of the present invention also provides vertical waveguidesolution for light-emitting devices grown on light absorbing substrates.Light absorbing substrates usually have a larger refractive index thanthat of the light-emitting device grown thereon. For example, this caseincludes nitride-based visible LEDs grown on silicon, gallium arsenide,and silicon carbide substrate. Substrate 10 can be one of these lightabsorbing substrates.

The lattice structure according to some embodiments of the presentinvention include plurality of rods which implement vertical waveguideeffect, because of its lower refractive index (say, sapphire, n=1.76)compared to that of the n-layer, the p-layer and the light-emittinglayer of a LED (say, GaN, n=2.46). These vertical wave-guiding rodsblock the horizontal waveguide effect arising from the refractive indexmismatch between the epilayers (e.g., the n-layer, the p-layer and thelight-emitting layer of a LED) and the substrate. In conventional LEDstructures, horizontal dimension is much larger than vertical dimension.For example, the vertical thickness of an LED structure is only a fewmicrons, where the horizontal chip size can be up to millimeters. Thetransformation of horizontal waveguide effect into vertical waveguideeffect greatly reduces self-absorption loss. At the meantime, thevertical waveguide effect enabling light to strike at the medium/airinterface with a small angle of incidence, eliminates total internalreflection, and results in enhanced light extraction efficiency.

The etching depth, or the height of rods 12 or sidewalls 12′ can be inthe range of 1 to 10 microns, being selected to make the rods/sidewallsin the vicinity of the light-emitting layer. In some embodiments, therods/sidewalls barely punch through the light-emitting layer and barelypunch into the p-type hole supplier layer. The sequential p-type layer'sgrowth planarizes the whole structure. In other embodiments, therods/sidewalls also punch through the p-type hole supplier layer. Thecross-section area of rod 12/sidewall 12′ in the top view direction canbe made small enough, to make sure that growth of the n-type layer andthe light-emitting layer only occurs in the etched area, not on top ofthe rods/sidewalls, due to sufficient adatom surface diffusion lengthunder epitaxy temperatures. In some embodiments according to thisinvention, when the vertical waveguide is made of vertical rods 12(refer to FIGS. 1, 12 and 13), the cross-section dimension (d) of rod 12typically is in the range of 500 nm to 5 μm, but, it is not limitedthereto. Especially, the dimension (d) can be made as small as possible,for example, between 200 and 400 nm, but smaller dimension (d) willincrease the manufacturing cost. The cross-section shape of rod 12 canbe of circle, square, triangle, pentagon, or hexagonal shape, or thelike. The cross-section dimension (d) of rod 12 may be substantially thesame along the vertical direction, or may increase or decrease upwardfrom the substrate along the vertical direction as shown in FIGS. 16-18.Rod 12 may also have a varying composition, thus, a varying refractiveindex, along the vertical direction, for example, a gradually reducingor increasing refractive index from bottom to top of rod 12.

The side surface of rods 12 of the lattice structure can besubstantially perpendicular to the substrate, or inclined within a smallangle with respect to the substrate surface normal. The inclined angleis preferably in the range of −15 to 15 degrees. The surfaces of rods 12are preferably to be optically smooth, though rough surfaces can also beimplemented in some embodiments.

The lattice structure of rods 12 can be of any two-dimensional lattice,or arranged randomly. For example, it can be a square lattice (refer toFIG. 12). In FIG. 12, round rods 12 forming a square lattice, with therod cross-sectional diameter, d, being from 500 nm to 5 μm, latticeconstant, a, being from 2 d to 10 d, i.e., from 1 to 50 μm. The latticestructure can also be a general rhombus lattice. In FIG. 13 a specialrhombus lattice, hexagonal lattice is presented. Here the latticeconstant and cross-sectional diameter fall into the same range mentionedabove. Rods 12 can also be randomly arranged.

In another aspect of the present invention, the lattice structure ismade of sheet-shaped sidewalls 12′ (refer to FIGS. 5-11) penetratingthrough the n-type electron supplier layer and the light-emitting layer.In some embodiments according to the present invention, the thickness(d) of sidewall 12′ is from 200 nm to 2 μm. But, it is can be thinnerthan 200 nm or thicker than 2 μm. The period (a) of the latticestructure of sidewalls 12′ can be, but is not limited to, from 5 to 50μm (FIGS. 5, 6, 10, and 11), The sidewalls 12′ of the lattice structurecan be substantially perpendicular to the substrate 10, or inclinedwithin a small angle with respect to the substrate surface normal. Theinclined angle is preferably in the range of −15 to 15 degrees. Thesurface of sidewalls 12′ are preferably to be optically smooth, thoughrough surface can also be implemented in some embodiments. The thickness(d) of sidewall 12′ may be substantially the same along the verticaldirection, or may increase or decrease upward from the substrate alongthe vertical direction as shown in FIGS. 16-18. Sidewall 12′ may alsohave a varying composition, thus a varying refractive index, along thevertical direction, for example, a gradually reducing or increasingrefractive index from bottom to top of sidewalls 12′.

Sidewalls 12′ and rods 12 can be fabricated in the same manner throughthe same or similar process. Therefore, rods 12 shown in FIGS. 1A-11E,2A-2C, 3A-3F, 4A-4B, and 14-18 can be replaced by sheet-shaped sidewalls12′.

Sidewalls 12′ of the lattice structure divide the light-emitting layerand the n-type layer (also the p-type layer if sidewalls 12′ punchthrough the p-type layer) into a plurality of area units such as areaunit 12′-i as shown in FIGS. 5 and 6, arbitrarily or in certainpatterns.

The area units can be closed area units, i.e., the area units aretotally separated from each other by sidewalls 12′. Therefore, lighttraveling horizontally in an area unit will be blocked by the sidewall12′ surrounding that area unit, which significantly reduces the distancetraveled by the light within the light-emitting layer, the n-type layer,or the p-type layer before it is extracted from the light-emittingdevice. As a result, the horizontal waveguide effect is significantlysuppressed and the absorption loss is greatly reduced. An example of theclosed area units are shown in FIG. 5. For example, each repeatingsquare surrounded by sidewall 12′ as shown in FIG. 5 represents a closedarea unit 12′-i. The area unit can have the shape of rhombus, pentagon,hexagonal, or any other shapes that can fill the two-dimensional spaceby translation, or any other random shapes.

The area units can be semi-closed area units, i.e., the area units arepartially separated from each other by sidewalls 12′, but still indirect connection with each other. Some examples of the semi-closed areaunits are shown in FIGS. 6-11. Therefore, light traveling horizontallyin an area unit will be mostly blocked by the sidewall 12′ surroundingthat area unit, only a small portion of the light travels horizontallyout of an area unit and enters the adjacent area units, which can alsosignificantly reduce the average distance traveled by the light withinthe light-emitting layer, the n-type layer, or the p-type layer beforeit is extracted from the light-emitting device. A feature of thesemi-closed area units is that electrical connection, for example in then-type layer, between the area units is maintained. A semi-closed areaunit can be obtained by having a gap “s” in sidewall 12′ as shown inFIG. 6. A lattice structure of semi-closed area units can also be formedby area units randomly divided by sidewalls 12′.

If each area unit 12-i is viewed as a cylindrical or disk-shaped body,the sidewall of the cylindrical or disk-shaped body is completelysurrounded by sidewall 12′ of the lattice structure for a closed areaunit, while the sidewall of the cylindrical or disk-shaped body is onlypartially surrounded by sidewall 12′ for a semi-closed area unit,leaving a portion of the sidewall of the cylindrical or disk-shaped bodyuncovered by the sidewall 12′. The enclosure degree of an area unit 12-iis defined as the ratio between the area of the sidewall of thecylindrical or disk-shaped body that is covered by the sidewall 12′ andthe total area of the sidewall of the cylindrical or disk-shaped body.Preferably, the enclosure degree is equal to or larger than 50%, morepreferably equal to or larger than 70%. In some embodiments according tothe present invention, the enclosure degree is designed in the rangebetween 20%-50%. In some embodiments according to the present invention,the enclosure degree is designed in the range between 80%-95%. When theenclosure degree approaches to 100%, the area unit becomes a closed areaunit.

Another factor to be considered about the lattice structure of robs12/sidewalls 12′ is the filling factor which is defined as the ratiobetween the volume occupied by the sidewalls 12′ or rods 12 in thelight-emitting layer and the total volume of the light-emitting layer.The filling factor preferably is in the range of 8-20%, and can be madebelow 8% as long as a minimum thickness of sidewalls 12′ is obtainableand still able to function as intended.

The rods 12 or sheet-shaped sidewalls 12′ of the lattice structure canalso be made from materials that are different from the substrate andwith smaller, or similar, or large refractive indexes compared to thatof the light-emitting layer. Candidate materials include, but notlimited to, epoxy resin (n=1.4), SiO₂/glass (n=1.46), sapphire (n=1.76),ITO (n=1.8), SiN_(x) (n=2.1), group-III nitride (AlInGaN, n>=2.2), TiO₂(n=2.6), GaP (n=3.3), ZnO, and other transparent metal oxides. Thismeans that instead of etching a substrate as shown in FIGS. 1A-1E, thesubstrate is first coated with a single layer of material or multiplelayers of materials listed above or other suitable materials. In FIG.2A, on a substrate 10, a single layer or multiple layers of material 11are deposited to a thickness greater than 1 μm, more preferably greaterthan 4 μm. Then per the protection of mask 15 and ICP-RIE etching oflayer 11, the remaining portions of layer 11 forms rods 12, or sidewalls12′, or the mixture of the two in FIG. 2B. Similar to the process stepsin FIGS. 1D and 1E, epilayer structure and device are formed in FIG. 2Cwhich include an insulating buffer layer 21, an n-type electron supplierlayer 22, a light-emitting layer 23, a p-type hole supplier layer 24, atransparent p-type contact layer 32, and an n-type contact layer 31. Thevertical height of rods 12 or sidewalls 12′ is in the range of 1 to 10microns, being selected to make the rods 12/sidewalls 12′ penetrate thelight-emitting layer 23. In some embodiments, the rods 12/sidewalls 12′penetrate the n-type layer 22 and barely punch through thelight-emitting layer 23. In this case, the growth of the p-type holesupplier layer 24 planarizes the structure. In some embodiments, therods 12/sidewalls 12′ also punch through p-type layer 24. In this case,the additional transparent p-type contact layer 32, such as ITO,planarizes the structure.

When layer 11 in FIG. 2A is formed by multiple layers of materials, rods12/sidewalls 12′ with grading refractive index along vertical directioncan be obtained. This will further enhance the vertical light extractionefficiency. In some embodiments, rods 12/sidewalls 12′ are made to havea refractive index above 2.46 in the vicinity of the light-emittinglayer or the light-emitting layer 23, by depositing TiO₂ or GaP, and tohave a decreasing refractive index as getting further way from thelight-emitting layer 23, by depositing AlGaN, SiN_(x), ITO, and SiO₂ inturn. By choosing multiple layers with different refractive index, or bychoosing a single layer doped with different concentration or type ofdopants, a desirable profile of refractive index along the verticaldirection of the light emitting device can be achieved.

FIGS. 3A-3F illustrate another method of making vertical light guidingstructure. It differs from FIGS. 1A-1E in that the rods 12/sidewalls 12′are coated, by growth and re-etching, with a high-quality insulatinglayer such as GaN or AlGaN layer on their vertical walls. To obtain thehigh-quality insulating layer, epitaxial growth of a nitride layer orother suitable material layer is conducted on rods 12/sidewalls 12′ oversubstrate 10. This high-quality coating serves the LEDs better undercertain circumstances. Firstly, it greatly reduces interface defectsbetween the rods 12/sidewalls 12′ and the light-emitting structure whichincludes the light-emitting layer, the n-type layer and the p-typelayer. Secondly, it can extract light out of the light-emittingstructure, into the coating layer, further reducing the self-absorptionlight loss. FIGS. 3A-3B are identical to FIGS. 1B-1C. In FIG. 3C, ahigh-quality insulating layer 14 of GaN, AlGaN, or InGaN is epitaxiallydeposited on rods 12/sidewalls 12′ over the etched substrate 10. Afterplanarization, a further etch step is perform to etch away most of theinsulating layer 14 leaving only a thin conformable layer 14′, forexample with a thickness of 200 nm to 1000 nm, on the rods 12/sidewalls12′ (FIG. 3D). Conformable layer 14′ covers rods 12/sidewalls 12′ and,may or may not, also cover substrate 10. When conformable layer 14′covers substrate 10, it can be made to function as an insulating bufferlayer for epitaxial growth of n-type electron supplier layer 22 or othersuitable layer thereon. If a portion of conformable layer 14′ is removedto expose substrate 10, an optional buffer layer, such as buffer layer21 as shown and described in FIG. 1D, can be formed on substrate 10 forthe growth of n-type electron supplier layer 22 thereon. In this way, alattice of composite rods 12 and/or sidewalls 12 is achieved. LED growthis then performed on the etched substrate with the composite rods12/sidewalls 12′. The composite rods 12/sidewalls 12′ have small enoughcross-section in the top view direction, ensuring that growth of then-type electron supplier layer 22, the light-emitting layer 23, andother layers penetrated by rods 12/sidewalls 12′ only happens in theetched area, not on top of the rods 12/sidewalls 12′, due to sufficientadatom surface diffusion length under epitaxy temperatures. The etchdepth, or the height of rods 12/sidewalls 12′ is selected to make therods 12/sidewalls 12′ penetrate the light-emitting layer 23. In someembodiments, the rods 12/sidewalls 12′ penetrate the n-type electronsupplier layer 22 and barely punch through the light-emitting layer 23.In this case, the growth of p-type layer 24 planarizes the structure. Insome embodiments, bigger refractive index contrast exists within thecomposite rods/sidewalls, and smaller refractive index contrast existsbetween the light-emitting layer 23 and the conformable layer 14′, lightis confined in the conformable layer 14′ and vertically guided to theemitting surfaces. Hence, light is extracted immediately away from thelight-emitting layer after its generation, avoiding the strongself-absorption of the light-emitting layer and horizontal waveguideeffect. These embodiments can result in even better enhanced lightextraction efficiency.

An alternative way of forming insulating conformable layer 14′ as shownin FIGS. 3A-3F is to coat rods 12/sidewalls 12′ by sputtering, notepitaxial growth, with one or more transparent layers possessing similaror higher refractive index than that of the light-emitting layer 23. Inthis case, the sputtering coated insulating conformable layer 14′ can bemade of gallium-containing nitride, silicon nitride (n=2.3), TiO₂(n=2.6), or the like.

Still referring to FIGS. 3A-3F, in this embodiment, light-emittingdevices are grown on a light absorbing substrate 10, which has a largerrefractive index than that of n-type layer 22, light-emitting region 23,and p-type layer 24. For example, this case includes nitride-basedvisible LEDs grown on silicon, gallium arsenide, and silicon carbidesubstrate. In this embodiment, the insulating layer 14 has a smallerrefractive index compared to that of n-type layer 22, light-emittingregion 23, and p-type layer 24. In the case of nitride-based visibleLEDs grown on Si, SiC, GaAs, and the like, insulating layer 14 can bemade of epoxy resin (n=1.4), SiO₂ (n=1.46), or Al₂O₃ (n=1.76) with athickness comparable to the emitted light wavelength in the medium (200nm to 1000 nm). Insulating layer 14 can be deposited on rods 12/sidewall12′ by any conventional depositing method and converted into aconformable layer 14′ by etching. In this way, a lattice structure ofcomposite rods 12/sidewalls 12′ is achieved. The epitaxial growth ofn-type layer 22, light-emitting region 23, or p-type layer 24 is thenperformed on the etched substrate with the composite rods 12/sidewalls12′. Conformable layer 14′ can also be replaced by a gap filled with air(n=1), which means there is a substantial gap between the rods12/sidewalls 12′ and n-type layer 22 and light-emitting region 23. Thegap can be formed by removing conformable layer 14′ after forming n-typelayer 22, light-emitting region 23, or p-type layer 24 (if rods12/sidewalls 12′ also penetrate through p-type layer 24).

In these embodiments, bigger refractive index contrast exists betweenthe light-emitting medium and the coating layer of the compositerods/sidewalls of the lattice structure, light is vertically confined inthe light-emitting medium and is vertically guided to the emittingsurfaces, avoiding the strong self-absorption due to horizontalwaveguide effect. These embodiments can result in enhanced lightextraction efficiency.

Conformable layer 14′ can be similarly applied to the light emittingstructure as shown in FIGS. 2A-2C.

FIGS. 4A-4B present another embodiment of the present invention.Substrate 10 can be of any kind of suitable materials conventionallyused in the field, such as sapphire, spinel, quartz, gallium nitride,silicon carbide, silicon, gallium arsenide, and the like. A thin bufferlayer 21 similar to that as shown and discussed in FIGS. 1A-1E isfirstly deposited on substrate 10 to prepare for the followingepilayers' growth. Over the buffer layer 21, a thick conducting layer 26is deposited. Conducting layer 26 provides a good electrical conductionto the layers or devices deposited thereon. Conducting layer 26 can beformed over buffer layer 21 by epitaxial growth or by other conventionalmethod. In the nitride-based visible LED cases, conducting layer 26 ispreferably Si-doped GaN, with a thickness greater than 2 μm, and a sheetresistance less than 30 Ω/cm. Conducting layer 26 can also be formedusing other conductive materials such as Si-doped InGaN or AlGaN. Uponconducting layer 26 an electrically insulating layer 28 is deposited.The thickness of layer 28 is determined depending on the thickness ofn-type layer 22, the light-emitting layer 23, or p-type layer 24 andsome other factors. For example, the thickness of insulating layer 28can the same as or slightly larger than the sum of the thickness ofn-type layer 22 and light-emitting layer 23, or the same as or slightlylarger or slightly smaller than the sum of the thickness of n-type layer22, light-emitting layer 23 and p-type layer 24. In some embodiments,the thickness of insulating layer 28 is in the range from 0.5 to 1.0 μm.In some embodiments, the thickness of insulating layer 28 is in therange from 1.0 to 10 μm. Further, referring to FIG. 4B, insulating layer28 is etched to form rods 12 or sidewalls 12′ and, then, epitaxialgrowth of n-type layer 22, light-emitting layer 23, and p-type layer 24is performed in a similar way to the steps shown in FIGS. 1A-1E, exceptthat, now the etching depth is controlled by the interface of insulatinglayer 28 and conducting layer 26. Here etching is fulfilled with fullelectrical access to conducting layer 26 by the later formed n-typelayer 22. In other words, n-type layer 22 or each area unit of n-typelayer 22 defined by sidewalls 12′ should be in electrical connectionwith conducting layer 26. In some embodiments, over-etching 0.1-0.3 μminto conducting layer 26 is conducted to make sure devices thereon havefull electrical access to conducting layer 26. However, the over-etchinginto conducting layer 26 cannot excess the thickness of conducting layer26. Again, rods rods 12/sidewalls 12′ can be coated with a layer ofmaterial, such as silicon nitride, TiO₂, or other materials as discussedabove. Since now the device is sitting on the thick conducting layer 26and the n-type layer 22 is electrically connected to the conductinglayer 26, the closed area units of n-type layer 22 are electricallyconnected to each other through conducting layer 26, those closed areaunits otherwise would not electrically connected to each other.

The lattice structure of rods 12/sidewalls 12′ can be of any one and/ortwo dimensional lattice, or randomly arranged. Exemplary lattices shownin FIG. 5 to FIG. 13 and others discussed above can be applied to theembodiments illustrated in FIGS. 4A-4B.

Presented in FIG. 5 is a top view of a two-dimensional lattice structuremade of sheet-shaped sidewalls 12′, showing sidewalls 12′ forming asolid grid which defines a plurality of closed area units 12′-i. It isunderstood from FIGS. 4A-4B and FIG. 5 that the sidewalls 12′ completelyencircle each closed area unit 12-i in the light-emitting layer 23,preventing light leaking horizontally across the light-emitting layer23. It is believed that this will block the horizontal waveguide effectto a maximal degree and implement a preferably vertical light extractioneffect. Alternatively, shown in FIG. 10-11 is a one-dimensional latticestructure. The dislocated one-dimensionally zigzagged sidewalls 12′ cansignificantly reduce horizontal waveguide effect, and have a similareffect to enhance the vertical light extraction efficiency. Thethickness, d, of the sidewalls 12′ in FIG. 5 to FIG. 11 is preferably inthe range of from 200 nm to 5 μm, more preferably from 500 nm to 2 μm.And the period, a, is preferably to be from 5 to 50 μm.

In addition, the present invention can be applied to devices grown onpatterned, or roughened substrate, for a further enhanced vertical lightextraction efficiency. This is shown as an exemplary embodiment in FIG.14, where rods 12 or sidewalls 12′ together with a patterned substrate10′ give a good light extraction effect. In the patterned substrate 10′,a plurality of vertical light waveguide structures 40 are provided fromthe bottom side opposite to the light-emitting layer. Structure 40 canbe a structure similar to that of rods 12/sidewalls 12′ in terms ofmaterial, shape, and dimension, formed by etching substrate 10′ andfilling the etched portion with proper solid material or with air.Structure 40 can be formed by any proper conventional method such asthose described in U.S. Pat. No. 7,504,669, U.S. Pat. No. 7,642,108, andU.S. Pat. No. 7,250,635, the contents of these patents are incorporatedherein by reference in their entirety.

In some embodiments, multiple sets of vertically arranged latticestructures of rods 12/sidewalls 12′ are applied to enhance the verticalwaveguide effect. Shown in FIG. 15 are two sets of lattice structures12-1 and 12-2 vertically positioned relative to each other, wherelattice structure 12-1 is embedded in insulating buffer layer 21 andn-type electron supplier layer 22, while lattice structure 12-2penetrates an auxiliary n-type electron supplier layer 22′ and the lightemitting layer 23, and extends up to or into the p-type hole supplierlayer 24. The two lattice structures can be vertically aligned ordislocated, and their lattice parameters can be the same or different.With two or more vertically arranged lattice structures, the height ofrods 12 and sidewalls 12′ in each lattice structure can be reduced. Thismakes it easier to fabricate the lattice structure, especially when thethickness of the sidewalls 12′ or the rods 12 is very thin. The heightof sidewalls 12′ and rods 12 in each of the multiple lattice structurescan be in the range 0.5 to 1.5 μm.

The light-emitting device shown in FIG. 15 can be fabricated as follows.Lattice structure 12-1 is formed on substrate 10 via any methoddescribed above in connection with FIGS. 1A-1E, 2A-2C, 3A-3F, and 4A-4B.Lattice structure 12-1 can be formed from substrate 10, or from a layerof material different than substrate 10 such as layer 11 as shown inFIG. 2A-2C and layer 28 as shown in FIG. 4A-4B. Epitaxial growth ofinsulating layer 21 and n-type electron supplier layer 22 is conducteduntil the top of lattice structure 12-1 is covered by the n-typeelectron supplier layer 22, Then, a layer of material such as layer 11as shown in FIG. 2A-2C and layer 28 as shown in FIG. 4A-4B is formed onn-type electron supplier layer 22, and etched to form lattice structure12-2 and to expose the n-type electron supplier layer 22. The auxiliaryn-type electron supplier layer 22′ is formed on n-type electron supplierlayer 22, then light-emitting layer 23 and p-type hole supplier layer 24are formed. Auxiliary n-type electron supplier layer 22′ can be made ofa material the same as or different from that of the n-type electronsupplier layer 22. Transparent p-electrode 32 and n-electrode 31 arealso formed accordingly. In this light-emitting device structure, thethickness of n-type electron supplier layer 22 can be in the range of0.5 to 5 μm, the thickness of auxiliary n-type electron supplier layer22′ can be in the range of 0.5 to 2 μm. Similarly, more than twovertically arranged lattice structures can be provided to further reducethe height of rods 12 and sidewalls 12′.

Lattice structure 12-1 and lattice structure 12-2 can be inclinedrelative to substrate 10 at an angle as shown in FIGS. 16-18, suchincline angle can be larger than 60° for convenience of fabrication.

The present invention has been described using exemplary embodiments.However, it is to be understood that the scope of the present inventionis not limited to the disclosed embodiments. On the contrary, it isintended to cover various modifications and similar arrangement orequivalents. The scope of the claims, therefore, should be accorded thebroadest interpretation so as to encompass all such modifications andsimilar arrangements and equivalents.

What is being claimed is:
 1. A method for fabricating a light-emittingdevice comprising: patterning and etching a substrate to form a latticestructure extending upward from the substrate; forming a n-type layerover the substrate so that the lattice structure penetrates the n-typelayer; forming a light-emitting layer over the n-type layer so that thelattice structure penetrates the light-emitting layer; and forming ap-type layer over the light-emitting layer so that the lattice structureextends up to the p-type layer or into the p-type layer.
 2. The methodof claim 1, further comprising forming a coating on the latticestructure before forming the n-type layer, the light-emitting layer, andthe p-type layer.
 3. The method of claim 2, wherein the coating isformed via epitaxial growth and etching back, or via sputtering, or viaoxidation.
 4. The method of claim 2 further comprising removing thecoating after forming the n-type layer and the light-emitting layer, sothat a gap is formed between the lattice structure and the n-type layerand the light-emitting layer.
 5. The light-emitting device of claim 1,wherein the lattice structure comprises a plurality of rods whichpenetrate the n-type layer and the light-emitting layer, and extend upto or into the p-type layer.
 6. A method for fabricating alight-emitting device comprising: forming a material layer over asubstrate; patterning and etching the material layer to form a latticestructure from the material layer, wherein the lattice structure extendsupward from the substrate; forming a n-type layer over the substrate sothat the lattice structure penetrates the n-type layer; forming anlight-emitting layer over the n-type layer so that the lattice structurepenetrates the light-emitting layer; and forming a p-type layer over thelight-emitting layer so that the lattice structure extends up to or intothe p-type layer.
 7. The method of claim 6, wherein the material layercomprises multiple layers of different refractive index.
 8. The methodof claim 6, wherein the material layer has a varying refractive index ina direction perpendicular to the substrate.
 9. The method of claim 6,further comprising forming a conductive layer between the material layerand the substrate before patterning and etching the material layer,wherein the step of patterning and etching the material layer alsoetches into a top portion of the conductive layer.
 10. A method forfabricating a light-emitting device comprising: patterning and etching asubstrate to form a first lattice structure extending upward from thesubstrate; forming a first n-type layer over the substrate so that thelattice structure is embedded in the first n-type layer; forming amaterial layer over the first n-type layer; patterning and etching thematerial layer to form a second lattice structure from the materiallayer; forming a second n-type layer over the first n-type layer so thatthe second lattice structure penetrates the second n-type layer; forminga light-emitting layer over the second n-type layer so that the secondlattice structure penetrates the light-emitting layer; and forming ap-type layer over the light-emitting layer so that the second latticestructure extends up to the p-type layer or into the p-type layer.
 11. Amethod for fabricating a light-emitting device comprising: forming afirst material layer over a substrate; patterning and etching thematerial layer to form a first lattice structure from the materiallayer, wherein the lattice structure extends upward from the substrate;forming a first n-type layer over the substrate so that the firstlattice structure is embedded in the first n-type layer; forming asecond material layer over the first n-type layer; patterning andetching the material layer to form a second lattice structure from thematerial layer; forming a second n-type layer over the first n-typelayer so that the second lattice structure penetrates the second n-typelayer; forming a light-emitting layer over the second n-type layer sothat the second lattice structure penetrates the light-emitting layer;and forming a p-type layer over the light-emitting layer so that thesecond lattice structure extends up to the p-type layer or into thep-type layer.